The present invention relates to a charge detecting signal processing circuit, and more particularly to a charge detecting signal processing circuit utilizing a source-follower circuit.
In recent years, a charge coupled device has widely been used to various devices and machines such as digital cameras, facsimile machines and copy machines. The requirement for improvement in characteristics and performances of the charge coupled device for improvements in the image quality has been on the increase. In order to improve the image quality, it is effective to reduce a dark random noise of the charge coupled device. The dark noise means signal variations which cause variations in level of the same pixel for individual signals on individual pixel lines when no light is irradiated onto an image sensor of the charge coupled device. One factor causing the dark noise is a thermal noise which is generated from an output buffer provided in the charge coupled device. The thermal noise is due to a thermal motion at random of charges. The thermal noise has a flat frequency spectrum which has a contact level over entire frequency ranges.
A source-follower circuit is provided in the charge coupled device, wherein the source-follower circuit serves as the output buffer which may generate the thermal noise. FIG. 1 is a circuit diagram illustrative of a first conventional source-follower serving as an output buffer in a charge coupled device.
The first conventional source-follower has a reset plus terminal φ R, and an output terminal Vout. The first conventional source-follower is connected between a power line VDD and a ground line. The first conventional source-follower has a series connection of a first enhancement type n-channel MOS field effect transistor 701 and a second enhancement type n-channel MOS field effect transistor 702 between the power line and the ground line. The output terminal Vout is connected to an intermediate point between the first enhancement type n-channel MOS field effect transistor 701 and the second enhancement type n-channel MOS field effect transistor 702, so that the first enhancement type n-channel MOS field effect transistor 701 is connected in series between the power line VDD and the output terminal Vout, whilst the second enhancement type n-channel MOS field effect transistor 702 is connected in series between the output terminal Vout and the ground line. The first enhancement type n-channel MOS field effect transistor 701 has a gate connected to a first node N1 which is further connected to a floating diffusion amplifier FDA. The second enhancement type n-channel MOS field effect transistor 702 has a gate connected to a second node N2. Further, the first conventional source-follower has a series connection of a first resistance 704 and a second resistance 705 between the power line VDD and the ground line. The first resistance 704 is connected in series between the power line VDD and the second node N2, whilst the second resistance 705 is connected in series between the second node N2 and the ground line. The first resistance 704 and the second resistance 705 form a voltage dividing circuit, so that the second node N2 has a second potential V2. A depletion type n-channel MOS field effect transistor 703 is connected in series between the first node NI and the power line VDD. The depletion type n-channel MOS field effect transistor 703 has a gate connected to a reset plus terminal, into which a reset plus φ R is inputted. A charge transferred to the floating diffusion amplifier FDA is reset upon application of the reset pulse φ R to the gate of the depletion type n-channel MOS field effect transistor 703. The second enhancement type n-channel MOS field effect transistor 702 serves as a current source and also the voltage dividing circuit is provided for controlling the current and the off-set voltage of the source follower circuit. The thermal noise is generated by the voltage dividing circuit. The thermal noise may be inputted into the gate of the second enhancement type n-channel MOS field effect transistor 702, thereby increasing random noises at the output terminal Vout of the source follower circuit. The first enhancement type n-channel MOS field effect transistor 701 may be considered to be the driver transistor, whilst the second enhancement type n-channel MOS field effect transistor 702 may be considered to be the load transistor.
The thermal noises appearing at the output terminal Vout of the source follower circuit may be classified into the following three noises. The first type thermal noise (Vn1) is generated from the n-channel MOS field effect transistors in the source-follower circuit. The second type thermal noise (Vn2) appears at the output terminal Vout of the source-follower circuit upon input of a noise into the gate of the first enhancement type n-channel MOS field effect transistor 701 from the floating diffusion amplifier FDA. The third type thermal noise (Vn3) appears at the output terminal Vout of the source-follower circuit upon input of a noise into the gate of the second enhancement type n-channel MOS field effect transistor 702 from the second node “N2” as the output of the voltage dividing circuit. The first, second and third noises are caused from independent noise sources from each other. A noise voltage Vno caused by the thermal noise appearing at the output terminal of the source follower circuit is given by:Vno=√{square root over ( )}{(Vn1)2+(Vn2)2+(Vn3)2}  (1)where Vn1 is the first type thermal noise, Vn2 is the second type thermal noise, and Vn3 is the third type thermal noise. The first type thermal noise Vn1 is the noise generated from the source-follower circuit. The second type thermal noise Vn2 is the noise inputted into the source-follower circuit. Those noises are not directly relevant to the issue of the present invention.
There will hereinafter be considered the thermal noise voltage Vno2 supplied from the gate of the second enhancement type n-channel MOS field effect transistor 702. The thermal noise generated by the resistance is given by:
 Vn=√{square root over ( )}(4kTRΔf)  (2)
where k is the Boltzmann's contact, T is the absolute temperature, R is the resistance value and Δf is the noise band width. It is assumed that the first resistance 704 has a first resistance value R1 and the second resistance 705 has a second resistance value R2. A resistance between the power voltage line VDD and the second node N2 as the output terminal of the voltage dividing circuit is a parallel resistance to the first resistance 704. A resistance between the ground line GND and the second node N2 as the output terminal of the voltage dividing circuit is a parallel resistance to the second resistance 705. A composite resistance value R is given by:R=(R1×R2)/(R1+R2)  (3)
The above third equation (3) is incorporated into the above second equation (2) to obtain the thermal noise voltage Vno2 which is generated from the voltage dividing circuit, wherein the thermal noise voltage Vno2 is given by:Vno2=√{square root over ( )}[4kT{(R1×R2)/(R1+R2)}Δf)  (4)
The third type thermal noise Vn3 appearing at the output terminal Vout of the source follower circuit is caused by the noise voltage Vno2 which is generated from the voltage dividing circuit. FIG. 2 is a circuit diagram illustrative of a modified conventional source-follower circuit from the above first conventional source-follower circuit, wherein the noises are inputted into the gate of the second enhancement type n-channel MOS field effect transistor. A first enhancement type n-channel MOS field effect transistor 301 corresponds to the above first enhancement type n-channel MOS field effect transistor 701. A second enhancement type n-channel MOS field effect transistor 302 corresponds to the above second enhancement type n-channel MOS field effect transistor 702. The first enhancement type n-channel MOS field effect transistor 301 and the second enhancement type n-channel MOS field effect transistor 302 are connected in series between the power voltage line VDD and the ground line GND. The first enhancement type n-channel MOS field effect transistor 301 is connected in series between the power voltage line VDD and the output terminal Vout. The second enhancement type n-channel MOS field effect transistor 302 is connected in series between the ground line GND and the output terminal Vout. The gate and drain of the first enhancement type n-channel MOS field effect transistor 301 is connected to the power voltage line VDD, wherein it is considered that the reset transistor turns ON and the gate of the first enhancement type n-channel MOS field effect transistor 301 has the same potential as the power voltage line VDD. The source of the first enhancement type n-channel MOS field effect transistor 301 is connected to the source follower output terminal Vout. The gate of the second enhancement type n-channel MOS field effect transistor 302 is connected to the second node. The drain of the second enhancement type n-channel MOS field effect transistor 302 is connected to the source follower output terminal Vout. The source of the second enhancement type n-channel MOS field effect transistor 302 is connected to the ground line. A noise inputted into the gate of the first enhancement type n-channel MOS field effect transistor 301 is ignored, whilst another noise inputted into the gate of the second enhancement type n-channel MOS field effect transistor 302 is considered. This circuit configuration is the same as an n-channel MOS inverter circuit. The noise voltage Vn3 appearing at the source-follower output terminal Vout is given by a product of the above second thermal noise voltage Vno2 of the dividing circuit and a gain of the n-channel MOS inverter. The n-channel MOS inverter shown in FIG. 2 has a gain Av. Assuming that a transmission conductance of the n-channel MOS field effect transistor is sufficiently larger than a transmission conductance caused by a substrate bias and also than a channel conductance caused by a channel modification effect, then the gain Va of the n-channel MOA inverter is given by:Av=−(gm2/gm1)  (5)where “gm1” is the transmission conductance of the first enhancement type n-channel MOS field effect transistor 301, and “gm2” is the transmission conductance of the second enhancement type n-channel MOS field effect transistor 302. The noise voltage Vn3 appearing at the source follower output terminal Vout and being caused by the thermal noise voltage Vno2 of the dividing circuit is given by:Vn3=√{square root over ( )}[{−(gm2/gm1)Vno2}2]  (6)
It is effective for reducing the thermal noise caused by the dividing circuit to increase “gm1” and decrease “gm2”. It is possible to reduce the transmission conductance “gm2” of the second enhancement type n-channel MOS field effect transistor 302 in the load side. However, the increase in the transmission conductance “gm1” of the first enhancement type n-channel MOS field effect transistor 301 in the driver side results in deterioration in the detective capacity of the floating diffusion amplifier, for which reason it is difficult to sufficiently increase the transmission conductance “gm1” of the first enhancement type n-channel MOS field effect transistor 301. It is thus possible that the thermal noise caused by the dividing circuit is relatively large in ratio to a total noise of the source-follower circuit. There has been used a second conventional source-follower circuit, wherein a gate of a transistor in a load side is free from an input of noises. FIG. 3 is a circuit diagram illustrative of a second conventional source-follower circuit. The second conventional source-follower circuit has a reset plus terminal φ R, and an output terminal Vout. The second conventional source-follower is connected between a power line VDD and a ground line. The second conventional source-follower has a series connection of a first enhancement type n-channel MOS field effect transistor 801 and a second enhancement type n-channel MOS field effect transistor 802 between the power line and the ground line. The output terminal Vout is connected to an intermediate point between the first enhancement type n-channel MOS field effect transistor 801 and the second enhancement type n-channel MOS field effect transistor 802, so that the first enhancement type n-channel MOS field effect transistor 801 is connected in series between the power line VDD and the output terminal Vout, whilst the second enhancement type n-channel MOS field effect transistor 802 is connected in series between the output terminal Vout and the ground line. The first enhancement type n-channel MOS field effect transistor 801 has a gate connected to a first node N1 which is further connected to a floating diffusion amplifier FDA. The second enhancement type n-channel MOS field effect transistor 802 has a gate connected to the ground line, so that no noise is supplied to the gate of the second enhancement type n-channel MOS field effect transistor 802. Further, the second conventional source-follower has no voltage dividing circuit. A depletion type n-channel MOS field effect transistor 803 is connected in series between the first node N1 and the power line VDD. The depletion type n-channel MOS field effect transistor 803 has a gate connected to a reset plus terminal, into which a reset plus φ R is inputted. A charge transferred to the floating diffusion amplifier FDA is reset upon application of the reset pulse φ R to the gate of the depletion type n-channel MOS field effect transistor 803. Since no voltage dividing circuit is provided, no thermal noise is inputted into the gate of the second enhancement type n-channel MOS field effect transistor 802, thereby causing no random noises at the output terminal Vout of the source follower circuit. The noise appearing at the output terminal Vout of the source-follower circuit is given by subtracting Vn3 from the above equation (1). The first enhancement type n-channel MOS field effect transistor 801 may be considered to be the driver transistor, whilst the second enhancement type n-channel MOS field effect transistor 802 may be considered to be the load transistor.
The second conventional source-follower circuit utilizes the depletion type n-channel MOS field effect transistor 802 in place of the voltage dividing circuit, thereby reducing the thermal noise as compared to the first conventional source-follower circuit. Variation in threshold voltage of the depletion type field effect transistor caused by the diffusion process is larger by about five times than variation in threshold voltage of the enhancement type field effect transistor caused by the diffusion process. The variation in threshold voltage of the depletion type field effect transistor caused by the diffusion process may be in the range of −1V to +1V. As a result, the input output characteristics of the source-follower circuit may have large variations for the following reasons. In recent years, the most of the charge coupled devices uses a buried-channel charge coupled device. In order to reduce the number of the fabrication processes, the channel of the depletion type field effect transistor is formed in the same time process as the buried channel of the charge coupled device. A thermal diffusion process is carried out to form an n-well region over a p-type substrate for the purpose of forming the buried channel of the charge coupled device. If the channel of the depletion type field effect transistor is formed in the same time process as the buried channel of the charge coupled device, then such the depletion type field effect transistor is larger in threshold voltage variation than the depletion type field effect transistor formed in the normal processes for the following reasons. The process for forming the buried channel of the charge coupled device is higher in dose than the normal process for forming the channel of the depletion type field effect transistor. The process for forming the buried channel of the charge coupled device is also longer in diffusion time period than the normal process for forming the channel of the depletion type field effect transistor. The higher dose and longer diffusion time of the process for forming the buried channel of the charge coupled device cause a larger variation in impurity profile. The larger variation in impurity profile causes the larger variation in threshold voltage. The larger variation in threshold voltage causes a larger variation in input-output characteristics of the source-follower circuit.
In the above circumstances, it had been required to develop a novel circuit for processing charge detecting signals free from the above problem.